Printing inks for offset and/or high printing containing nir absorbers and nir absorbers soluble in offset and/or high printing inks

ABSTRACT

Printing inks for offset and/or letterpress printing which comprise NIR absorbers, and solubility of the NIR absorber in the printing ink is at least 0.1% by weight. NIR absorbers consisting of cyanine cation with an anion which has long-chain alkyl or aralkyl groups. Use of such printing inks for printing processes in which the curing of the printing ink is promoted by using IR lasers. NIR absorbers comprising a cyanine cation with an anion which has long-chain alkyl or aralkyl groups.

The present invention relates to printing inks for offset and/orletterpress printing which comprise NIR absorbers and where thesolubility of the NIR absorber in the printing ink is at least 0.1% byweight. In a particular embodiment, the NIR absorber comprises a cyaninecation with an anion which has long-chain alkyl or alkylaryl groups. Theinvention furthermore relates to the use of such printing inks inprinting processes in which the curing of the printing ink is promotedby using IR radiation sources. In a further aspect, the inventionrelates to an NIR absorber comprising a cyanine cation with an anionwhich has long-chain alkyl or alkylaryl groups.

Curing or drying of printing inks is understood by a person skilled inthe art as meaning the entire group of operations, reaction sequencesand transformations which take place in the transformation of theprinting ink freshly applied to the print medium into a solid filmadhering well to the substrate. Substantial processes here are—in thecase of an absorptive print medium—the penetration of the solvent intothe print medium, the evaporation of the solvent and the crosslinking ofthe film, for example oxidatively by means of atmospheric oxygen or by afree radical or cationic method by means of suitable crosslinkingagents.

Faster curing permits faster printing and thus increases thecost-efficiency. It is known that the curing of printing inks can alsobe accelerated by supplying heat. For this purpose, for example, thefreshly printed print medium can be passed through a drying tunnel andheated with hot air and/or IR emitters. The procedure is usual,particularly in offset printing, because offset printing inks comprisevery high-boiling solvents which exhibit virtually no evaporation atroom temperature. For example, the heatset (roll) offset method iswidely used. Further details in this context are to be found, forexample, in Römpp-Lexikon “Lacke and Druckfarben”, Georg Thieme Verlag,Stuttgart, New York 1998, page 279/280.

IR emitters generally emit broad-band radiation both in the near and inthe middle infrared range. As an alternative to the use of IR emitters,EP-A 355 473 or EP-A 1 302 735 has proposed the use of lasers, inparticular IR lasers, for heating the print layer. Conventional IRlasers emit in particular in the near infrared. Of course, the curing ofthe printing ink layer with IR radiation is all the better the higherthe power density of the radiation. This is why lasers are particularlysuitable.

Disadvantageously, however, the IR radiation is absorbed not only by theprinting ink on the print medium but also by the paper. In particular,water is important as an IR absorber in the paper. Small amounts ofwater are always present in the paper, for example as a result ofabsorption of atmospheric humidity. Furthermore, water also enters thepaper via the fountain solution used for offset printing. If the paperis heated excessively due to strong absorption and dries outnonuniformly, it may become wavy in an undesired manner. This problemhas been discussed in more detail, for example in sections [0010],[0011] and [0012] in EP-A 1 302 735.

In order to solve the problem, EP-A 1 302 735 proposed using radiationenergy sources which emit at a wavelength which is substantially notabsorbed by water. In order nevertheless to ensure sufficient absorptionof the IR radiation in the printing ink layer, it was proposed to useprinting inks which comprise an absorber for NIR radiation. Only twocyanine dyes in the form of the perchlorate and tetrafluoroborate, an aminimum compound in the form of the perchlorate and a nickel-dithiolenecomplex were mentioned specifically.

Cyanine dyes and their preparation are known in principle, for examplefrom DE-A 37 21 850, and they have been proposed for a very wide rangeof applications, for example for photographic materials (U.S. Pat. No.5,013,642, EP-A 342 576, EP-A 445 627), ink ribbons (DE 43 08 635) orprinting plates (WO 03/66338). Cyanine dyes are commercially available.

Cyanine dyes consist of a cyanine cation and a corresponding anion. Thismay be an anion which is present separately or an internal anion, i.e.the anionic group is chemically bonded to the cyanine cation. In theirpreparation, they are usually obtained as simple salts, for example ashalides, tetrafluoroborates, perchlorates or tosylates. Cyanine dyeshaving anions which have long-chain alkyl groups or alkyl-substitutedaryl groups have been unknown to date.

However, the use of said simple salts in offset printing inks leads toproblems. If a sufficient amount of the NIR absorbers are stirred intothe printing inks, the hue of the offset printing ink changes. Thiseffect is highly undesirable since the tristimulus values of a set ofprinting inks, yellow, cyan, magenta and black, for high-qualityfour-color printing are tailored to one another with high precision andspecified by means of international standards. Even very smalldeviations from the CIE coordinates are no longer acceptable inhigh-quality offset printing. The change in the hue is most noticeablein the case of yellow, which becomes dull on addition of such NIRabsorbers and acquires a brownish-greenish tinge. Such a yellow iscompletely unusable.

The change in the hues appears to be caused at least partly byinsufficient solubility of the dyes in the offset printing inks. Thesolubility of conventional cyanine dyes in the nonpolar, viscoussolvents which are used for offset and letterpress inks is as a ruleless than 0.1%.

It was therefore an object of the invention to provide improved printinginks for offset and/or letterpress printing which comprise NIR absorbersand where the disadvantages of the prior art are not observed. It was afurther object to provide NIR absorbers which are suitable for thepreparation of printing inks for offset and/or letterpress printing andwhich can be stirred into the printing inks and, in the printing ink,lead to a sufficiently high extinction in the case of certainwavelengths typical for lasers, without any substantial impairment ofthe tristimulus values of the printing ink occurring.

Accordingly, offset printing inks which comprise NIR absorbers have beenfound, and the solubility of the NIR absorber in the ink is at least0.1% by weight, the solubility of the NIR absorber being greater than orequal to the respective concentration of the NIR absorber in theprinting ink.

In a preferred embodiment of the invention, the NIR absorber is an ionicabsorber comprising a cyanine cation X⁺and a corresponding anion1/mY^(m−), the cyanine cation having a general formula (I) or (II)

-   -   n is 1 or 2 and the radicals R¹ to R⁹ have the following        meanings:    -   R¹ and R², independently of one another, are a linear or        branched, optionally further substituted alkyl or aralkyl        radical having 1 to 20 carbon atoms,    -   R³ and R⁴, independently of one another, are H or CN,    -   R⁵ and R⁶, independently of one another, are one or more,        identical or different substituents selected from the group        consisting of —H, —F, —Cl, —Br, —I, —NO₂, —CN, —CF₃, —R¹, —OR¹,        aryl- or —O-aryl,    -   R⁷ is —H, —Cl, —Br, —I, -phenyl, —O-phenyl, —S-phenyl,        —N(phenyl)₂, -pyridyl, a barbituric acid radical or a dimedone        radical, it also being possible for the phenyl radicals to be        further substituted,    -   R⁸ and R⁹, independently of one another, are >C(CH₃)₂, —O—, —S—,        >NR¹ or —CH═CH—,    -   and the anion Y^(m−) has the general formula [AR¹⁰ _(k)]^(m−)        with a polar, ionic head group A and k nonpolar groups R¹⁰, k is        1, 2 or 3 and m is 1 or 2, and the nonpolar groups R¹⁰,        independently of one another, are selected from the group        consisting of    -   linear, branched or cyclic alkyl groups having 6 to 30 carbon        atoms and    -   alkylaryl groups of the general formula -aryl-R¹¹, where R¹¹ is        a linear or branched alkyl group having 3 to 30 carbon atoms,        or the anion Y^(m−) is a borate anion of the general        formulae (V) or (VI)

where R¹⁰ is as defined above and R¹² is at least one substituentselected from the group consisting of H and linear, cyclic or branchedalkyl groups having 1 to 20 carbon atoms, andin the radicals R¹⁰, R¹¹ and R¹², even nonneighboring carbon atoms mayoptionally be substituted by O atoms and/or the radicals R¹⁰, R¹¹ andR¹² may be completely or partly fluorinated, with the proviso that thenonpolar character of the group is not substantially influenced thereby.

Novel NIR absorbers of the type described have furthermore been found.

Regarding the invention, the following may be stated specifically:

The novel offset printing inks comprise, in a manner known in principle,at least one nonpolar solvent, a binder and a colorant absorbing in thevisible spectral range. In addition, conventional additives may bepresent.

The terms “offset printing ink” and “letterpress printing ink” areself-explanatory and at the same time limiting. Letterpress printinginks are also known as relief printing inks.

Offset and letterpress printing inks are in each case pasty printinginks which comprise high-boiling, nonpolar solvents, as a rule having aboiling point of about 200° C. to about 320° C. The general principlesfor the formulation of offset and letterpress printing inks are known toa person skilled in the art and are described, for example, in referenceworks such as Römpp-Lexikon “Lacke and Druckfarben”, Georg ThiemeVerlag, Stuttgart, New York 1998, or Leach, Robert H.; Pierce, Ray J.“The Printing Ink Manual”, 5th Ed.—London, Blueprint, 1993.

The novel printing inks can in principle be all types of offset and/orletterpress printing inks. However, a heatset offset printing ink ispreferred.

The novel printing ink comprises, in a manner known in principle, atleast one nonpolar, high-boiling solvent. Of course, mixtures ofdifferent solvents may also be used provided that the properties of theprinting inks are not adversely affected thereby. Examples of suitablesolvents comprise mineral oils, in particular low-aromatics mineraloils. The boiling point of the mineral oil depends on the desired useand is chosen accordingly by a person skilled in the art. In general, aboiling point of about 200° C. to about 270° C. is advisable for heatsetoffset printing, and a boiling point of about 240° C. to 320° C. forcoldset offset printing and letterpress printing. Further examplescomprise vegetable, semidrying or drying oils, such as, for example,soybean oil, wood oil, tall oil or linseed oil. Such oils are suitablein particular for sheet-fed offset and letterpress printing inks. Theyare preferably used as a mixture with mineral oils.

The person skilled in the art makes a suitable choice from the solventsdepending on the desired properties of the printing ink. The sameapplies to the amount of the solvent used. In particular, amounts offrom 5 to 45% by weight, based on the amount of all components of theprinting ink, of solvent have proven useful, without there being anyintention to limit the invention thereto.

The novel printing inks furthermore comprise, in a manner known inprinciple, at least one binder. Mixtures of different binders arepreferably used, for example mixtures of hard resins and soft resins.The conventional binders typical for offset and letterpress printinginks may be used. Examples of suitable binders comprise alkyd resins,natural resins, such as rosins, which may also be modified, such as, forexample, phenol- or maleate-modified rosins, or synthetic resins, suchas, for example, coumarone, indene or cyclopentadiene resins. Dependingon the application, amounts of from about 20 to 70% by weight, based onthe amount of all components of the printing ink, have proven useful,without there being any intention to limit the invention thereto. Theperson skilled in the art suitably chooses the type and amount of thebinder according to the desired properties of the printing ink.

The novel printing ink furthermore comprises colorants absorbing in thevisible spectral range. The conventional colorants known for offset andletterpress printing inks, in particular conventional pigments, may beused. Examples are inorganic pigments, such as, for example, titaniumdioxide pigments or iron oxide pigments, interference pigments, carbonblacks, and organic pigments, such as azo, phthalocyanine andisoindoline pigments. The colorants may also be soluble organic dyes. Itis of course also possible to use mixtures of different colorants. Theamount of colorant is usually 5-25% by weight, based on the sum of allcomponents of the printing ink.

The novel printing inks can optionally comprise, in a manner known inprinciple, one or more assistants or additives. Examples of additivesand assistants are fillers, such as calcium carbonate, hydrated aluminumoxide or aluminum or magnesium silicate. Waxes increase the abrasionresistance and serve for reducing the blocking resistance. Examples arein particular polyethylene waxes, oxidized polyethylene waxes, petroleumwaxes or ceresin waxes. Fatty acid amides can be used for increasing thesurface smoothness. Dispersants can be used for dispersing the pigments.Cobalt or manganese salts, i.e. drying agents, can be used foraccelerating oxidative curing. The total amount of all additives andassistants usually does not exceed 20% by weight, based on the sum ofall components, and is preferably 0.1-10% by weight.

According to the invention, the printing inks for letterpress and/oroffset printing further comprise at least one NIR absorber which hassubstantially no absorption in the visible spectral range. It is ofcourse also possible to use a plurality of different NIR absorbers.

NIR absorbers are also referred to by a person skilled in the art as NIRdyes or, more generally, as IR dyes. Such dyes or absorbers haveabsorption maxima in the spectral range from 700 nm to 3000 nm,preferably from 750 nm to 2000 nm, particularly preferably from 780 nmto 1500 nm.

In the context of this invention, the term “substantially no absorptionin the visible spectral range” is intended to mean that the absorbershould ideally have no absorption at all in the visible spectral range.For the purposes of this invention, however, it is sufficient if theabsorption of the NIR absorber—in the chosen amounts—in the visiblespectral range is so low that the color impression of the printing inkis not adversely affected. Of course, this also depends on the hue andon the color strength of the respective printing ink. An NIR absorberwhich is no longer suitable for a printing ink having a very specifichue and a very specific color strength may in certain circumstances beentirely suitable for another printing ink having another hue andanother color strength.

According to the invention, the solubility of the NIR absorber in theprinting ink is at least 0.1% by weight, based on the sum of allcomponents of the ink, with the proviso that the solubility of the NIRabsorber is greater than or equal to the concentration of the NIRabsorber in the printing ink.

In other words, it must be ensured according to the invention that addedIR absorber is completely dissolved in the printing ink. The larger theadded amount of the NIR absorber, the greater must also the solubilityof the added NIR absorber be. Depending on the desired amount of IRabsorber, the person skilled in the art makes a suitable choice from theNIR absorbers possible in principle, taking particular account of thesolubility thereof. The solubility can of course also depend on theprinting ink. An NIR absorber which is not sufficiently soluble in aspecific printing ink may have sufficient solubility in another one.

The type and amount of the NIR absorber present in the novel printingink is chosen by a person skilled in the art so that sufficientabsorption at the desired laser wavelength is achieved. As a rule, anamount of less than 5% by weight is sufficient. An amount of from 0.05to 4% by weight, based on all components of the printing ink, preferablyfrom 0.1 to 3% by weight, particularly preferably from 0.2 to 2.5% byweight and very particularly preferably from 0.3 to 2.0% by weight, hasproven particularly useful.

The solubility of the NIR absorber in the printing ink is preferably atleast 0.2% by weight, particularly preferably at least 0.5% by weight,very particularly preferably at least 1.0% by weight and, for example,at least 2% by weight.

As a rule, it is advisable not to increase the amount of added NIRabsorber to the solubility limit but to remain a certain distance awayfrom the solubility limit.

Any desired NIR absorbers can be used by a person skilled in the art forthe preparation of the printing ink, provided that the NIR absorber hasthe required solubility. However, the NIR absorber is preferably atleast one NIR absorber selected from the group consisting of thecyanines, naphthalocyanines, squaraines and croconates.

In a particularly preferred embodiment of the invention, the NIRabsorber is an ionic absorber comprising a cyanine cation X⁺ and acorresponding anion 1/mY^(m−), where m can assume in particular thevalues 1 or 2.

The cyanine cation according to the invention has a general formulaselected from the following formulae (I) to (IV):

Here, n is 1 or 2, and the radicals R¹ to R⁹ have the followingmeanings:

R¹ and R², independently of one another, are a linear or branched alkylor aralkyl radical having 1 to 20 carbon atoms. Examples comprisemethyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl, tert-butyl,1-pentyl, 1-hexyl, 2-ethyl-1-hexyl, 1-octyl, 1-decyl or 1-dodecylgroups. In particular, they are linear alkyl groups. Preferred radicalsare methyl, ethyl, 1-butyl or 1-dodecyl groups. Aralkyl groups are, in amanner known in principle, alkyl groups substituted by aryl groups.Examples comprise a benzyl or phenylethyl group. R¹ and R² may beidentical or different from one another. R¹ and R² are preferablyidentical radicals.

R¹ and R² may optionally be further substituted. In particular,functional groups, such as, for example, amino or hydroxyl groups, maybe mentioned here. If present, they may be in particular functionalgroups arranged terminally on alkyl groups.

R³ and R⁴, independently of one another, are —H or —CN. R³ and R⁴ arepreferably the same group.

The radicals R⁵ and R⁶ are different or, preferably identical radicalsselected from the group consisting of —H, —F, —Cl, —Br, —I, —NO₂, —CN or—CF₃. R⁵ and R⁶ may also be a radical —R¹ or —OR¹, where R¹ in each casehas the above meaning. Furthermore, they may be aryl or —O-arylradicals, aryl preferably being a phenyl radical. R⁵ and R⁶ arepreferably —H, —Cl, —Br or —I or an alkyl radical. The terminal ringsmay each also have a plurality of identical or different substituents R⁵or R⁶ at different positions of the ring.

Preferably not more than two substituents are present on each ring,particularly preferably only one substituent in each case.

R⁷ may be —H, —Cl, —Br, —I, -phenyl, —O-phenyl, —S-phenyl, —N(phenyl)₂,-pyridyl, a barbituric acid radical or a dimedone radical, it also beingpossible for the phenyl radicals to be further substituted. Furthersubstituents may be, for example, straight-chain or branched alkylradicals, for example methyl or ethyl radicals, or —F, —Cl, —Br, —I,—NO₂, —CN or —CF₃.

The radicals R⁸ and R⁹ are different or, preferably, identical radicalsselected from the group consisting of >C(CH₃)₂, —S—, >NR¹ or —CH═CH—.They are particularly preferably >C(CH₃)₂.

The opposite ion Y^(m−) to the cyanine cation may have the generalformula [AR¹⁰ _(k)]^(m−). It comprises a polar, ionic head group A and knonpolar groups R¹⁰, where k is 1, 2 or 3 and m is 1 or 2. The anionpreferably has only one group R¹⁰. It is furthermore preferably amonovalent anion. If a plurality of nonpolar groups R¹⁰ are present inthe anion, they may be different or, preferably, identical. Of course, amixture of a plurality of different anions is also possible.

The groups R¹⁰ may be linear, branched or cyclic alkyl groups having 6to 30 carbon atoms. The alkyl groups R¹⁰ preferably have 6 to 12 carbonatoms. Examples of suitable groups comprise 1-hexyl, cyclohexyl,2-ethyl-1-hexyl, 1-octyl, 1-nonyl, 1-decyl, 1-undecyl, 1-dodecyl or1-tetradecyl groups. They are preferably linear alkyl groups.

They may furthermore be alkylaryl groups of the general formula-aryl-R¹¹, R¹¹ being a linear or branched alkyl group having 3 to 30carbon atoms. Examples of suitable groups comprise 1-propyl, 2-propyl,1-butyl, 2-butyl, tert-butyl, 1-pentyl, 1-hexyl, cyclohexyl,2-ethyl-1-hexyl, 1-octyl, 1-nonyl, 1-decyl, 1-undecyl, 1-dodecyl- or1-tetradecyl groups. The alky groups R¹¹ preferably have 6 to 12 carbonatoms. They are particularly preferably linear alkyl groups. The arylunit is in particular a phenylene group, preferably a 1,4-phenylenegroup. Examples of suitable alkylaryl groups comprise —(C₆H₄)—C₃H₇,—(C₆H₄)—C₆H₁₃ or —(C₆H₄)—C₁₂H₂₅.

The polar ionic head group A is in particular the anion of a monobasicor dibasic acid radical. It may also be any desired inorganic or organicacid group. It is preferably a carboxyl group or a S-, P- orB-containing acid group. For example, it may be an acid group selectedfrom the group consisting of —SO₃ ⁻, —OSO₃ ⁻, —COO⁻, —PO₃ ²⁻, —OPO₃ ²⁻or (—O)(—O)PO₂ ⁻.

Examples of particularly suitable anions comprise alkanesulfonateshaving alkyl radicals, in particular linear alkyl radicals of 6 to 12carbon atoms, such as, for example, n-octanesulfonate, n-decanesulfonateor n-dodecanesulfonate, and 4-alkylbenzenesulfonates having alkylradicals of 6 to 12 carbon atoms, such as, for example,4-hexylbenzenesulfonate, 4-octylbenzenesulfonate,4-decylbenzenesulfonate or 4-dodecylbenzenesulfonate. These may also be,in a manner known in principle, industrial products which have adistribution of different alkyl radicals of different lengths.

The opposite ion Y^(m−) for the cyanine cation may also be a borateanion of the general formula (V) or (VI)

R¹⁰ is a radical as defined above. It is possible in each case for oneor two identical or different substituents to be present on each of thechelate ligands. Preferably, in each case one substituent is present.R¹² comprises in each case one or more identical or differentsubstituents selected from the group consisting of H or linear, cyclicor branched alkyl groups having 1 to 20 carbon atoms, preferably aradical having 2 to 12 carbon atoms. Preferably, only one alkyl group ispresent as a substituent. Such borate anions are obtainable, forexample, from boric acid and the corresponding dialcohol.

In the radicals R¹⁰, R¹¹ and R¹², it is also possible for nonneighboringcarbon atoms optionally to be substituted by O atoms and/or for theradicals R¹⁰, R¹¹ and R¹² to be completely or partly fluorinated,provided that the nonpolar character of the groups is not substantiallychanged thereby.

The preparation of the novel NIR absorbers can be effected by means ofdifferent methods. They can be prepared, for example, by means of atwo-stage process in which, in a first step, the cyanine cations aresynthesized with conventional anions, such as iodide, tetrafluoroborate,perchlorate or paratoluenesulfonate. Preparation methods are known to aperson skilled in the art. As an example, reference may be made to DE-A37 21 850, EP-A 627 660 and the literature cited there. NIR absorbersbased on cyanine are also commercially available.

In a second step, the conventional anions are then exchanged for thenovel anions Y^(m−) by means of a suitable method.

This can be effected, for example, by initially taking the startingmaterial together with the corresponding acid H_(m)Y in awater-immiscible organic solvent, there being no need for the absorberto be soluble therein: Readily volatile organic solvents having acertain polarity are particularly suitable. For example, said solventmay be dichloromethane. The organic solution or suspension is thenextracted with water until the original anion has been completelyremoved from the organic solution. The novel NIR absorber can beobtained by removing the solvent from the solution.

The preparation can also be carried out using acidic ion exchange resinsby dissolving the starting salt having a conventional anion in asuitable polar solvent, for example an alcohol, such as methanol orethanol, and adding the solution to the ion exchange column. Theabsorber cations are then eluted with a solution of the desired anion.The ion exchange can also be effected similarly to the process disclosedby WO 03/76518.

The novel NIR absorbers are readily soluble in offset printing inks. Thesolubility can be influenced by the choice of the anion and of thesubstituents on the cation. Relatively long alkyl chains as groups R¹⁰,R¹¹ or R¹² or as substituents on the cyanine generally also lead tobetter solubility.

The novel NIR absorbers have absorption maxima in the range from 700 nmto 1200 nm. Those dyes which have their absorption maximum close to theemission wavelength of conventional lasers, in particular semiconductordiode lasers, are preferred. Examples of typical laser wavelengthscomprise 750 nm, 785 nm, 810 nm, 835 nm, 855 nm, 955 nm, 980 nm,preferably 810 nm and 980 nm. The absorption maximum of the NIR absorbercan be influenced by a person skilled in the art in a manner known inprinciple, by the choice of the substituents on the cyanine cation.

As already described at the outset, the novel NIR absorbers havesubstantially no absorption in the visible spectral range. Theextinction coefficient in the range from 400 to 700 nm is generally lessthan 20%, preferably less than 10% and particularly preferably less than5% of the extinction coefficient at the incident laser wavelength.

Since, owing to their high mass-specific extinction coefficients, thenovel NIR absorbers advantageously have to be used only in small amountsin order to achieve the desired effects, the hue of the printing ink isnot changed or at least substantially not changed by the addition of theNIR absorbers.

The preparation of the novel letterpress or offset printing ink has noparticular features at all. It can be effected by the methods known inprinciple, by thorough mixing or dispersing of the components inconventional apparatuses, such as, for example, dissolvers, stirred ballmills or a three-roll mill. Here, the NIR absorbers can be mixed in likeother additives in the course of the preparation and dissolved in theprinting ink.

It is also possible to mix the novel NIR absorbers into prepared,commercial offset or letterpress printing inks. Here, it is advisable asa′ rule to predissolve the novel absorbers in a small amount of mineraloil and to add them as a concentrate to the offset ink.

By means of the novel NIR absorbers, printing inks are obtained whichcomprise a sufficient amount of NIR absorber in dissolved form and inwhich the hue of the printing ink is nevertheless not changed or atleast substantially not changed in comparison with that of a printingink without such an NIR absorber.

The printing inks can in principle be used for all techniques ofletterpress or offset printing. They are of course particularly suitablefor all printing techniques in which the drying of the ink is promotedby means of IR radiation, in particular heatset offset printing. Bymeans of the IR absorber, very rapid drying of the printing ink appliedto the print medium is achieved.

The IR radiation used for the drying may be either broadband radiation,narrowband radiation or laser radiation having a very specificwavelength. Particularly suitable lasers are the known lasers emittingin the NIR range, for example semiconductor diode laser or solid-statelasers, such as, for example, Nd/YAG lasers.

The novel printing inks are particularly suitable for printing processesin which the curing of the printing ink is promoted by using radiationenergy sources whose wavelength is not resonant with absorptionwavelengths of water. This technique is particularly valuable whenprinting on paper, cardboard or the like. A narrowband radiation source,in particular a laser, is preferably used for this purpose. Thisadvantageously ensures that the water present in the print medium—andhence also the print medium itself—is not heated or at least notsubstantially heated. Adverse effects which may be caused by the heatingof the print medium, such as, for example, formation of waves ordeformation of the print medium, are thus avoided. By means of the IRabsorber contained in the print layer, the printed layer is neverthelessheated in a targeted manner and thus cures more rapidly. Details of thistechnique and apparatuses required for this purpose are described indetail in EP-A 1 302 735, which is to be considered a part of thisdisclosure. The person skilled in the art chooses from the novel NIRabsorbers those which have the best absorption at the wavelength desiredin each case.

The novel NIR absorbers can of course be used not only for thepreparation of offset or letterpress printing inks but also for otherapplications, for example as readily soluble IR absorbers in finishes,in particular clear finishes, or for IR filters.

The examples which follow are intended to explain the invention in moredetail:

A. Synthesis of the NIR Absorbers

The novel NIR absorbers can be synthesized in a two-stage process. Inthe first stage, the synthesis of the cyanine cations havingconventional anions, such as, for example, iodide, is effected. Thesynthesis is known in principle to a person skilled in the art and canbe carried out by syntheses known from the literature, for example bythe methods of K. Vankataraman “The Chemistry of Synthetic Dyes”,Academic Press, New York, 1952, Vol. II, and H. Zollinger “ColorChemistry: Synthesis, Properties, and Applications of Organic Dyes andPigments”, Weinheim, Wiley-VCH, 2003.

In a second stage, the conventional anion is exchanged for a novelanion.

1. Stage of Synthesis of Cyanine Cations with Conventional Anions

The synthesis of the absorber2-[2-[2-[2-(1,3-dihydro-1-ethyl-3-3-dimethyl-2H-indol-2-ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-1-ethyl-3,3-dimethyl-3H-indoliumiodide (A1) is described below by way of example.

10 g (0.032 mol) of 3-ethyl-1,1,2-trimethylindolium iodide and 2.7 g(0.016 mol) of 3-hydroxymethylenecyclohex-1-enecarbaldehyde areinitially taken in a mixture of 105 ml of butanol and 45 ml of toluene.Heating is effected to 110° C., and the water formed is removed. Afterstirring for five hours, cooling to room temperature is effected. Afterthe solution has been evaporated down, methyl tert-butyl ether is added.The crystals formed are filtered off with suction and washed with methyltert-butyl ether. 9.4 g of crystals are obtained and are dried at 50° C.under reduced pressure (m.p. 235° C.).

In an analogous manner, other cyanine cations with conventional anionscan be synthesized using corresponding starting compounds. The NIRabsorbers A1 to A3 not according to the invention are listed in table 1.

TABLE 1 Synthesized NIR absorbers not according to the inventionCompound Structure Anion A1

I— A2

I— A3

I—

2. Stage of General Method for the Preparation of Novel NIR Absorbers byExchange of the Anion2-[2-[2-[2-(1,3-Dihydro-1-ethyl-3-3-dimethyl-2H-indol-2-ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-1-ethyl-3,3-dimethyl-3H-indoliumdodecylsulfonate (B1)

The compound was prepared as follows: 0.003 mol (1.6 g) of the NIRabsorber A1 is initially taken together with 0.009 mol (2.3 g) of sodiumdodecanesulfonate in 50 ml of dichloromethane. 50 ml of water are added,stirring is effected for 30 minutes at room temperature and finally thephases are separated. The organic phase is washed three times with 50 mlof water until iodide is no longer detectable with silver nitratesolution in the wash water. After drying of the organic phase withsodium sulfate, the solvent is distilled off and the residue is dried at50° C. under reduced pressure.

In an analogous manner, the following NIR absorbers were prepared usingother cyanine cations and corresponding salts of the desired anions. Thesynthesized novel NIR absorbers B1 to B10 are summarized in table 1. Forcomparative purposes, samples of the corresponding iodides were alsoretained in each case.

TABLE 2 Synthesized novel NIR absorbers Compound λmax [nm] Cyaninecation Anion B1 (prepared above) 786

B2 786

B3 810

B4 810

B5 832

B6 810

B7 762

B8 823

B9 676

B10 768

B. Testing of the Novel NIR Absorbers in Printing Inks:

Two conventional varnishes for the preparation of offset printing inkswere used for the test. Varnishes are prepared formulations comprisingbinder and solvent, but as yet without colorant. In this way, it ispossible to study the behavior of the NIR absorber alsospectroscopically without interference by the colorant.

A varnish for the preparation of heatset inks comprising about 45% byweight of a low-aromatics mineral oil (boiling range from 240 to 270°C.), about 45% by weight of a rosin-modified phenol resin and about 10%by weight of an alkyd resin was used, as well as a varnish for thepreparation of sheet-fed offset inks comprising about 45% by weight of alow-aromatics mineral oil (boiling range from 260 to 290° C.), about 45%by weight of a rosin-modified phenol resin and about 10% by weight oflinseed oil.

In each case defined amounts of the NIR absorbers were added to thebinder solutions and stirring was effected for at least 4 hours at 60°C. After the cooling of the samples, polarization microscopy was used totest whether the resulting liquids still comprise undissolved crystalsof the NIR absorber.

Furthermore, a thin layer of above liquids was produced and wasinvestigated spectroscopically. For this purpose, the above liquids werediluted 1:5 with dichloromethane, and the resulting solution was appliedto a microscope slide by means of a knife coater so that, afterevaporation of the dichloromethane, a layer of about 2 μm thickremained. An absorption spectrum (400-1000 nm) of this layer was thenrecorded after 2 hours.

Result:

The NIR absorbers A1, A2 and A3 not according to the invention werevirtually insoluble in both solutions (solubility in each case <<0.01%by weight).

With the novel NIR absorbers B1, B2, B3 and B4, on the other hand, clearsolutions without undissolved crystals were obtained even with at least2% by weight of the corresponding NIR absorber in each case.

The thin layer of the printing varnish containing 1% by weight of theNIR absorber gave an extinction of E<<0.01 at 786 nm in the case of thecompound A1 not according to the invention, while an extinction ofE=0.91 at 786 nm resulted in the case of the novel compound B1.

An extinction E of <<0.01 at 810 nm was obtained for the compound A2(1.0% by weight) not according to the invention, while an extinction Eof 0.83 at 810 nm was obtained with the novel sample B4 (1.0% byweight).

Testing of the NIR Absorbers in Yellow Offset Printing Ink COMPARATIVEEXAMPLES

In each case 0.5% by weight of the absorber A1 or A2 not according tothe invention was added to a commercial yellow heatset offset printingink, and the mixtures were thoroughly stirred. The NIR absorber did notdissolve in the offset ink and instead dispersions resulted.

Samples of the inks obtained were printed on paper.

In comparison with a comparative sample without NIR absorber, the printlayer on the paper had in each case a brownish green hue instead of apure yellow hue.

The absorption at a laser wavelength of 786 nm or 810 nm is very low.

EXAMPLES

The procedure was as in the comparative example, except that in eachcase 0.5% by weight of the novel NIR absorber B2 or B4 was used. The NIRabsorbers B2 and B4 each dissolved completely in the offset ink.

In both cases, the print layer on the paper had a yellow hue, which wasunchanged in comparison with a sample without NIR absorber.

The absorption at the laser wavelength of 786 nm or 810 nm was high(>60%).

1-17. (canceled)
 18. A printing ink for letterpress and/or offsetprinting, comprising 5 to 45% by weight of at least one nonpolar solventwith a boiling point of from 200 to 320° C., 20 to 70% by weight ofbinder, 5 to 25% by weight of colorant absorbing in the visible spectralrange and an NIR absorber which has substantially no absorption in thevisible spectral range, wherein the solubility of the NIR absorber inthe printing ink is at least 0.1% by weight, based on all components ofthe printing ink, with the proviso that the solubility of the NIRabsorber is greater than or equal to the concentration of the NIRabsorber in the printing ink.
 19. The printing ink according to claim18, wherein the solubility of the NIR absorber is at least 0.2% byweight.
 20. The printing ink according to claim 18, wherein the NIRabsorber is at least one NIR absorber selected from the group consistingof cyanines, naphthalocyanines, squaraines and croconates.
 21. Theprinting ink according to claim 20, wherein the NIR absorber is an ionicabsorber comprising a cyanine cation X⁺ and a corresponding anion1/mY^(m−), the cyanine cation having a general formula selected from thegroup consisting of (I) to (IV)

where n is 1 or 2 and the radicals R¹ to R⁹ have the following meanings:R¹ and R², independently of one another, are a linear or branched,optionally further substituted alkyl or aralkyl radical having 1 to 20carbon atoms, R³ and R⁴, independently of one another, are H or CN, R⁵and R⁶, independently of one another, are one or more, identical ordifferent substituents selected from the group consisting of —H, —F,—Cl, —Br, —I, —NO₂, —CN, —CF₃, —R¹, —OR¹, aryl- and —O-aryl, R⁷ is —H,—Cl, —Br, —I, -phenyl, —O-phenyl, —S-phenyl, —N(phenyl)₂, -pyridyl, abarbituric acid radical or a dimedone radical, it also being possiblefor the phenyl radicals to be further substituted, R⁸ and R⁹,independently of one another, are >C(CH₃)₂, —O—, —S—, >NR¹ or —CH═CH—,and the anion Y^(m−) has the general formula [AR¹⁰ _(k)]^(m−) with apolar, ionic head group A and k nonpolar groups R¹⁰, k is 1, 2 or 3 andm is 1 or 2, and the nonpolar groups R¹⁰, independently of one another,are at least one selected from the group consisting of linear, branchedand cyclic alkyl groups having 6 to 30 carbon atoms and alkylaryl groupsof the general formula -aryl-R¹¹, where R¹¹ is a linear or branchedalkyl group having 3 to 30 carbon atoms, or the anion Y^(m−) is a borateanion of the general formulae (V) or (VI)

where R¹⁰ is as defined above and R¹² is at least one substituentselected from the group consisting of H and linear, cyclic and branchedalkyl groups having 1 to 20 carbon atoms, and in the radicals R¹⁰, R¹¹and R¹², even nonneighboring carbon atoms may optionally be substitutedby O atoms and/or the radicals R¹⁰, R¹¹ and R¹² may be completely orpartly fluorinated, with the proviso that the nonpolar character of thegroups is not substantially influenced thereby.
 22. The printing inkaccording to claim 21, wherein the polar, ionic head group A is at leastone monobasic or dibasic acid radical selected from the group consistingof —SO₃ ⁻, —OSO₃ ⁻, —COO⁻, —PO₃ ²⁻, —OPO₃ ²⁻ and (—O)(—O)PO₂ ⁻.
 23. Theprinting ink according to claim 21, wherein R¹⁰ is a linear, branched orcyclic alkyl group having 6 to 12 carbon atoms.
 24. The printing inkaccording to claim 23, wherein R¹⁰ is a linear alkyl group.
 25. Theprinting ink according to claim 21, wherein R¹¹ has 6 to 12 carbonatoms.
 26. The printing ink according to claim 25, wherein R¹¹ is alinear alkyl group.
 27. The printing ink according claim 18, wherein theamount of the NIR absorber in the printing ink is from 0.05 to 4% byweight, based on the sum of all components of the ink.
 28. A method ofcuring the printing ink according claim 18 in printing processes inwhich the curing of the printing ink is promoted by using IR radiationsources whose wavelength is not resonant with the absorption wavelengthsof water.